1. Field of the Invention
Embodiments of the invention relate to the field of circuit protection devices. More particularly, the present invention relates to a fuse employing a double wound fusible wire element configured to withstand high surge current associated with inductive and capacitive loads.
2. Discussion of Related Art
Fuses are typically used as circuit protection devices and form an electrical connection with a component in a circuit to be protected. The fuse is designed to protect the circuit or circuit component by being the intentional weak link in the circuit. One type of fuse includes a housing consisting of a plastic base and a plastic cap with a pair of conductors or terminals which extend through the base and are connected via a fusible element that forms a bridge between the terminals inside the housing. In order to fix the terminals inside the base portion of the housing, a portion of each terminal and/or the base is deformed in order to pinch the base around the terminals, thereby clamping the base around the respective terminals. The fusible element is attached to ends of each of the two conductors projecting above the base. The fusible element is typically a conductive wire which is soldered to the ends of the two terminals. The fuse is placed in a circuit to be protected such that the fusible element melts when an abnormal overload condition occurs.
In certain circuit protection applications (e.g. motors, etc.), a surge current or short term current overload situation may typically occur until a steady state condition for the device is achieved. Fuses employed in these types of circuits must be designed to permit this short term surge to pass through the fuse without melting the fusible element. This high-surge condition is defined in terms of current and time (I2t) where it is desirable to avoid an open circuit unless the current exceeds a specific percentage of the fuse's rated current.
One type of fuse used in these applications employs a spiral wound fuse element. In particular, the fuse element comprises a core of twisted yarn fibers with a fuse wire or wound around the core in a spiral pattern. The yarn that comprises the core is typically a ceramic material that is void of any material that could become conductive when the fuse is blown. The wound wire may include a plurality of wire strands configured to provide increased heat absorption indicative of, for example, a slow-blow or time-delayed fuse.
When a circuit overload is encountered, the passage of the excess current through the fuse element causes it to generate heat and thereby elevate the temperature of the fuse wire. In other words, the core acts as a heat sink to draw this heat away from the fuse wire, thereby lowering the temperature of the fuse wire. In this manner, the transfer of heat from the fuse wire to the core lengthens the time required before the fuse wire melting temperature is reached. For higher current-rated fuses, a larger diameter fuse wire is used to withstand higher current passing through the wire and therefore higher temperatures. However, the wound fuse wire is limited in size, thereby limiting the amount of excess current the wire can withstand as well as the amount of heat transfer between the wound wire and the core. Accordingly, there is a need for a fuse that utilizes a wound fusible wire element and a fuse employing the same configured to provide high I2t characteristics on the fuse element that will withstand high surge current associated with inductive and capacitive loads to protect particular types of circuit components and associated circuits.